At length he looked up at me.  “This is a very extraordinary affair, Jervis,” he said.

“Very,” I agreed; “and the question that is agitating me is, what is to be done?”

“Yes,” he said, meditatively, “that is the question; and an uncommonly difficult question it is.  It really involves the settlement of the antecedent question:  What is it that is happening at that house?”

“What do you think is happening at that house?” I asked.

“We must go slow, Jervis,” he replied.  “We must carefully separate the legal tissues from the medical, and avoid confusing what we know with what we suspect.  Now, with reference to the medical aspects of the case.  The first question that confronts us is that of sleeping sickness, or negro-lethargy as it is sometimes called; and here we are in a difficulty.  We have not enough knowledge.  Neither of us, I take it, has ever seen a case, and the extant descriptions are inadequate.  From what I know of the disease, its symptoms agree with those in your case in respect of the alleged moroseness and in the gradually increasing periods of lethargy alternating with periods of apparent recovery.  On the other hand, the disease is said to be confined to negroes; but that probably means only that negroes alone have hitherto been exposed to the conditions that produce it.  A more important fact is that, as far as I know, extreme contraction of the pupils is not a symptom of sleeping sickness.  To sum up, the probabilities are against sleeping sickness, but with our insufficient knowledge, we cannot definitely exclude it.”

“You think that it may really be sleeping sickness?”

“No; personally I do not entertain that theory for a moment.  But I am considering the evidence apart from our opinions on the subject.  We have to accept it as a conceivable hypothesis that it may be sleeping sickness because we cannot positively prove that it is not.  That is all.  But when we come to the hypothesis of morphine poisoning, the case is different.  The symptoms agree with those of morphine poisoning in every respect.  There is no exception or disagreement whatever.  The common sense of the matter is therefore that we adopt morphine poisoning as our working diagnosis; which is what you seem to have done.”

“Yes.  For purposes of treatment.”

“Exactly.  For medical purposes you adopted the more probable view and dismissed the less probable.  That was the reasonable thing to do.  But for legal purposes you must entertain both possibilities; for the hypothesis of poisoning involves serious legal issues, whereas the hypothesis of disease involves no legal issues at all.”

“That doesn’t sound very helpful,” I remarked.

“It indicates the necessity for caution,” he retorted.

“Yes, I see that.  But what is your own opinion of the case?”